Automatic trigger circuits for oscilloscopes have been in use for many years and are well known in the art under such names as "automatic time base," "bright baseline", and "peak-to-peak auto triggering". Such circuits in connection with a sweep generator typically provide a sweep that in the absence of a triggering signal free runs at a rate determined by its sweep rate and holdoff timing circuits. When a triggering signal is applied, the automatic triggering circuit reverts to a triggered mode in which time base sweep signals are produced in synchronism with the triggering signal.
Automatic triggering circuit schemes have included the use of triggerable free-running multivibrators, tunnel diode pulse generators, feedback systems, and logic circuits. Generally, such circuits are complex and require some knowledge of the circuit variables, such as input signal amplitude, voltage offset, triggering level adjustment, and slope control setting to attain a stable, recurrent display.